As a wireless communication system in which a wireless access point and a wireless terminal communicate with each other, the wireless LAN (Local Area Network) based on the IEEE 802.11 standard that uses CSMA/CA (Carrier Sense Multiple Access/Collision Avoidance) is widely known. IEEE Std 802.11™-2012 and IEEE Std 802.11ac™-2013 are known as wireless LAN specification. In the IEEE 802.11n standard, MIMO (Multi-Input Multi-Output) technology is used, which achieves an increased speed of wireless communication.
In the MIMO technology, a transmission-side apparatus and a reception-side apparatus each include a plurality of antennas installed therein. The transmission-side apparatus simultaneously transmits a plurality of pieces of data with the plurality of antennas. The reception-side apparatus simultaneously receives the plurality of pieces of data with the plurality of antennas, and separates the plurality of received pieces of data from one another. This allows a plurality of data streams to be transmitted and received. The higher the number of data streams is, the higher throughput can be obtained as compared with conventional SISO (Single-Input Single-Output).
In addition, in an effort to increase the speed, the IEEE 802.11ac standard is now being formulated. The IEEE 802.11ac standard uses a downlink multiuser MIMO (DL-MU-MIMO) technology, which is an expansion of the MIMO technology. In the downlink multiuser MIMO, an access point simultaneously transmits different pieces of data to a plurality of wireless terminals from a plurality of antennas. This allows the access point to simultaneously communicate with the plurality of wireless terminals.
The downlink multiuser MIMO uses a technique called beamforming. In the beamforming, a beam that causes data streams addressed to the wireless terminals to least interfere with one another, that is, a beam that makes data streams addressed to the wireless terminals spatially orthogonal, are formed and transmitted for each wireless terminal. This makes spatial multiplexing available, which in turn allows the access point to simultaneously transmit different pieces of data to the plurality of wireless terminals.
To implement the downlink multiuser MIMO, the access point needs to obtain information on channels between the antennas thereof and the wireless terminals. That is, the access point needs to detect how the wireless terminals receive signals transmitted from the antennas thereof. Using information on the channels allows the access point to form an optimal beam that does not interfere with other wireless terminals. The IEEE 802.11ac standard defines such a protocol that allows for estimation of the channels between the access point and the wireless terminals.
According to the protocol defined in the IEEE 802.11ac standard, the access point causes the wireless terminals to estimate channel responses and to feed them back in turn, so as to detect a state of the channels with respect to the wireless terminals. As a result, the higher the number of the wireless terminals performing channel estimation, the longer it takes for the estimation to be performed. This increases an overhead time taken to start the downlink multiuser MIMO. In addition, this decreases a correlation between a state of the channels when the wireless terminals estimated them and a state of the channels when the downlink multiuser MIMO is actually started. Thus, in the downlink multiuser MIMO transmission using the channel information obtained from the wireless terminals, an adequate beam may not be formed due to temporal fluctuations of the channels. Also, an inadequately formed beam may disadvantageously interfere with other wireless terminals.